The ongoing miniaturization of feature sizes in semiconductor manufacturing processes has facilitated the formation of microscopic structures, i.e. structures that have feature sizes in the micron and submicron, e.g. nanometer domain, on substrates such as silicon substrates. A prime example of such a microscopic structure is a microelectromechanical system (MEMS) structure. Such structures are sometimes also referred to as micromachines.
MEMS structures can be used for a wide range of applications in different fields of technology, e.g. electronics, medicine, pharmacy and chemistry. Applications in the field of electronics for instance include accelerometers, gyroscopes, sensors, and so on. The MEMS structures may be made from any suitable material, e.g. silicon, polymer, metals amongst others.
Typically, the MEMS structure requires a certain degree of translational freedom in order to perform its function. To this end, the MEMS structure is packaged such that the structure is located in a cavity. An example of such a package is for instance disclosed in Q. Li et al. in “Hermeticity and Thermal Stability Testing of PECVD Silicon Nitride Thin-Film Packages”, ICEP 2008, International Conference on Electronics Packaging, 10 Jun. 2008, Tokyo, Japan.
Silicon nitride is a particularly suitable material for such capping layers because of its strength (it is for instance used as anti-scratch material for ICs).
However, this paper discloses that a problem with plasma-enhanced chemical vapor deposited silicon nitride capping layers is that the deposition leads to significant hydrogen contamination in the silicon nitride. This causes problems when the package is subsequently exposed to thermal budgets in further processing steps, such as thermal budgets of around 450° C. At these temperatures, the hydrogen contamination tends to be released from the silicon nitride capping layer, which compromises the vacuum inside the cavity, thus impairing the functioning of the microscopic structure. This seriously hampers the further processing of such devices.
FIG. 1 schematically depicts the build-up of pressure inside such a cavity exposed to a temperature of 450° C. as a function of time (min.). It can be predicted from FIG. 1 using a double exponential model that an excess pressure of around 9 mTorr will be generated by the outgassing of hydrogen from the silicon nitride capping layer. This problem is exacerbated at higher temperatures, as is shown in FIG. 2, where the asymptotic excess pressure in mTorr as a function of anneal temperature for a 300 nm thick PECVD silicon nitride is depicted.